1. Field of the Invention
The present invention relates to integrated circuits and, more particularly, to voltage reference circuits.
2. Description of the Related Art
Precision voltage references are critical elements of various circuits, such as portable devices, instrumentation and test equipment, data acquisition systems, medical equipment, servo systems, and the like. Voltage reference circuits are used to supply a steady and reliable voltage reference to other circuitry or systems. Similarly, low dropout voltage (LDO) regulators are also used to provide regulated voltages in a precise and reliable manner. Recently, voltage references or regulators have begun to utilize complimentary metal-oxide-semiconductor (CMOS) technology.
FIG. 1 is a schematic diagram of a conventional voltage reference circuit 100 according to one embodiment of the invention. The conventional voltage reference circuit 100 includes a differential amplifier 102 that receives an input reference voltage (VREF) at one input terminal and receives a feedback voltage at another input terminal. An output terminal of the differential amplifier 102 is coupled to an output transistor 104. The output transistor 104 is typically a LDO power device. The conventional voltage reference circuit 100 also includes a resistor-capacitor network 106. The resistor-capacitor network 106 includes a load capacitor (CL) and resistors R1 and R2. The resistor-capacitor network 106 is provided between an output node 108 and ground potential. The feedback voltage is supplied to the differential amplifier 102 from a node 110 within the resistor-capacitor network 106. The load capacitor (CL) is required to be rather large (e.g., at least 1 xcexcF) so that loop stabilization results. The conventional voltage reference circuit 100 also receives an enable signal that is supplied to the differential amplifier 102. When the enable signal operates to xe2x80x9cenablexe2x80x9d the conventional voltage reference circuit 100, the output of the differential amplifier 102 activates the output transistor 104 to pull the output node 108 towards the power supply voltage (VDD) and thus produce a precise output reference voltage (VOUT). On the other hand, when the enable signal operates to xe2x80x9cdisablexe2x80x9d the differential amplifier 102, the output of the differential amplifier 102 deactivates the output transistor 104. In the disable situation, ideally the output voltage (VOUT) would immediately drop to ground potential. However, with respect to the conventional voltage reference circuit 100, the resistor-capacitor network is coupled to the output terminal 108 and thus, before the output voltage (VOUT) can be dropped to a near ground potential, the charge stored at the load capacitor (CL) needs to discharge through the resistors R1 and R2 to ground. This induces a RC time constant delay that slows the decay of the output voltage (VOUT) to near ground potential. Because of the rather large capacitance of the load capacitor (CL) and the non-trivial resistances of the resistors R1 and R2 (e.g., typically at least 10 k ohms), the RC time constant delay imposed causes the output voltage (VOUT) to slowly respond to the disable situation. Accordingly, while large load capacitors are used by conventional voltage reference circuits for loop stabilization, the large load capacitors hinder the rapid disabling of conventional voltage reference circuits. In some applications for voltage references or regulators, the failure to provide rapid disabling leads to undesirable effects. For example, in one common application, a voltage reference circuit is utilized to provide a precise voltage reference to an electrical system, such as a portable computing device. In such an application, when the voltage reference circuit is disabled, it is intended that the power to the electrical system be removed. However, the slow responsiveness of the output voltage (VOUT), when disabling the voltage reference circuit, causes the electrical system to undesirably consume power during the time it takes for the voltage reference circuit to become fully disabled (i.e., VOUTxcx9c=0). Accordingly, this leads to poor power management for the electrical system because until the voltage reference becomes fully disabled, the circuitry within the electrical system will continue to draw power rom a power source (e.g., a battery) of the portable computing device.
Thus, there is a need for reference-producing circuits that not only remain stable but also rapidly transition between enable and disable modes.
Broadly speaking, the invention relates to a reference-producing integrated circuit that is able to rapidly transition from an enable mode to a disable mode. The reference-producing integrated circuit can, for example, be a voltage reference integrated circuit or a voltage regulator. In one implementation, the voltage reference integrated circuit or the voltage regulator can provide a low dropout voltage output. The reference-producing integrated circuit is particularly useful for reducing power consumption by electrical circuitry (e.g., portable computing devices) being power managed at least in part through control of the voltage reference supplied to the electrical circuitry.
The invention can be implemented in numerous ways including as a method, a system, and a device. Several embodiments of the invention are discussed below.
As an integrated circuit, one embodiment of the invention includes at least: an amplifier, an output transistor, a load capacitor, a resistive element, and a discharge transistor. The amplifier produces an output signal based on a feedback voltage and a reference voltage. The output transistor having a gate terminal, a drain terminal and a source terminal, the gate terminal being operatively connected to receive the output signal, the drain terminal being operatively connected to an output terminal, and the source terminal being operatively connected to a first source voltage level. The load capacitor is operatively connected between the output terminal and a second source voltage level. The resistive element is operatively connected between the output terminal and the second source voltage level. The resistive element includes at least a series connection of first and second resistors, with the first resistor being operatively connected between the output terminal and a feedback node, and with the second resistor being operatively connected between the feedback node and the second source voltage level. The discharge transistor having a gate terminal, a drain terminal and a source terminal, the gate terminal being operatively connected to an enable signal supplied to the integrated circuit, the drain terminal being operatively connected to the output terminal, and the source terminal being operatively connected to the second source voltage level. The feedback voltage is provided to the amplifier by being operatively connected to the feedback node.
As a reference voltage integrated circuit for receiving a voltage reference input, an enable signal and a voltage reference output, one embodiment of the invention includes at least: a differential amplifier, the differential amplifier producing an output signal based on a voltage difference between a feedback voltage and the reference voltage input; an output transistor having a gate terminal, a drain terminal and a source terminal, the gate terminal being operatively connected to receive the output signal, the drain terminal being operatively connected to an output terminal, and the source terminal being operatively connected to a first source voltage level; a load capacitor operatively connected between the output terminal and a second source voltage level; a resistive element operatively connected between the output terminal and the second source voltage level, the resistive element including at least a series connection of first and second resistors, with the first resistor being operatively connected between the output terminal and a feedback node, and with the second resistor being operatively connected between the feedback node and the second source voltage level; and a discharge transistor having a gate terminal, a drain terminal and a source terminal, the gate terminal being operatively connected to the enable signal, the drain terminal being operatively connected to the output terminal, and the source terminal being operatively connected to the second source voltage level.
As a method for operating a voltage reference circuit to reduce power consumption in an electrical system, one embodiment of the invention includes the acts of: providing a voltage reference output from the voltage reference circuit to the electrical system, the power consumed by the electrical system being dependent on the voltage reference level; receiving a disable request to disable the voltage reference circuit; and thereafter, in response to the disable request, rapidly pulling the voltage reference output to about ground potential.
The advantages of the invention are numerous. Different embodiments or implementations may yield one or more of the following advantages. One advantage of the invention is that being able to provide a rapid disable of a voltage reference facilitates improved power management in associated circuitry. The improved power management can conserve a battery""s charge in cases of a battery powered, portable device. Another advantage of the invention is that overhead circuitry for providing rapid disable is minimal.
Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.